Developing cells progress through restricted molecular pathways that ultimately define a final endpoint. However, during times of stress, injury, or genetic alterations, cells exhibit remarkable plasticity in which they undergo metaplasia - a process by which one adult cell type is replaced by a different adult cell type. One organ that exhibits metaplasia is the pancreas, where conversion of epithelial acinar cells into ductal complexes (referred to as acinar-ductal metaplasia) occurs under a variety of physiological conditions. Acinar-ductal metaplasia is associated with an increased risk of neoplasia and is found in mouse models and in patients presenting with pancreatic ductal adenocarcinoma (PDA). Although activating mutations in the KRAS protooncogene (KrasG12D) are thought to initiate PDA development, our knowledge is grossly deficient in defining how KrasG12D expression leads to acinar-ductal metaplasia, in identifying transcriptional networks that are integral to advancing or repressing metaplasia, and in determining how cellular plasticity contributes to PDA development. In an effort to address these deficiencies, we have examined the importance of Mist1 - an acinar cell restricted basic helix-loop-helix transcription factor - to acinar-ductal metaplasia and pancreatic cancer. Using the Mist1 locus and the KrasG12D oncogene, several mouse models have been generated that provide an unique opportunity to study aspects of KrasG12D-induced metaplasia and to define the pancreatic cell lineages that participate in PDA formation. This proposal employs lineage tracing strategies, coupled with pancreatic cancer mouse models, to (i) characterize the lineage progression and molecular pathways that control acinar-ductal metaplasia in Mist1Kras/+ mice, (ii) define the importance of Mist1 to KrasG12D-induced cellular plasticity, and (iii) establish if acinar cell lineages, through acinar-ductal transdifferentiation events, generate pancreatic precancerous lesions. Successful completion of these aims will identify the developmental and molecular basis of the earliest events in acinar-ductal metaplasia and reveal how these actions impact the progression of pancreatic cancer. The data obtained from these studies will also determine the importance of Mist1 to metaplastic changes and establish if acinar-ductal transdifferentiation recapitulates earlier developmental transcriptional networks. Defining the molecular players that participate in adult cell plasticity will offer new therapeutic opportunities to reprogram aberrant cell growth and development. PUBLIC HEALTH NARRATIVE: Pancreatic cancer is the fourth leading cause of cancer deaths in the U.S., and despite a number of advances in basic and clinical pancreas biology, our understanding of how cells change their growth and differentiation characteristics remains unknown. In this application, studies are proposed to identify the gene regulatory networks that control the earliest events of pancreatic cancer development. Defining these events, and the genes that participate in altering adult cell phenotypes, will offer new therapeutic opportunities to reprogram aberrant cell growth in pancreatic cancer patients.